The present invention relates to a USB (Universal Serial BUS) interface system, and more particularly to a USB interface system and a device capable of supplying power from the device to a host.
USB is widely used as a general-purpose interface. USB is the standard standardized by the USB-IF (USB Implementers Forum) and it is a serial bus interface for connecting a host device such as a personal computer to a peripheral device such as a mouse and a keyboard. At present, USB is mounted in almost all the personal computers and they can be connected to any peripheral device if only having a USB terminal. USB includes two standards: USB 1.1 capable of transferring data in a low speed mode (1.5 Mbps) or in a full speed mode (12 Mbps) and USB 2.0 capable of transferring data at a high speed in a high speed mode (480 Mbps) in addition to the above-mentioned two modes. If USB 2.0 is employed, even a digital video can be connected to a computer.
A master-slave protocol is adopted in USB, and devices using the USB interface also have a master-servant relationship as a “host” and a “device”. Usually, a personal computer works as a host and a peripheral works as a device, and even when the peripherals such as a digital camera and a printer communicate with each other, the communication is established through the personal computer that is the host. When feeding power, it is designed to supply the power from the host to the device necessarily.
Recently, with the spread of portable devices such as portable phones and PDAs, and with variation and high performance of peripherals, there arises a demand of connecting the peripherals with each other. As mentioned above, however, it is impossible to directly connect a peripheral with another peripheral through USB. When connecting the peripherals with each other, at least one of the peripherals has to be provided with a function of the host.
In order to connect the peripherals directly with each other without interposing a personal computer therebetween, USB-OTC (USB On-The-Go) is released as an additional standard of the USB 2.0 (refer to Non-Patent Articles 1 and 2). In the USB-OTG, a dual role device is prepared, in which a peripheral can work as a host as well as a device. When a peripheral wants to receive power but communicate information as a host, a dual role device could act as a device at first, then reverse the role with receiving the power. Thus, it could act as a host while keeping the above state of receiving power. As a result, power supply from a device to a host can be realized.
[Non-Patent Article 1]
USB Implementers Forum, “USB-On-The-Go”, [online], [retrieval under the date of June 20, Hei-15], Internet<URL: http://www.usb.org/developers/onthego>
[Non-Patent Article 2]
David Duke, “Technical description: Additional standard “On-The-Go” for overcoming a problem of existing USB”, [online], CQ Publishing Co., Ltd., 2001, [retrieval under the data of June 20, Hei-15], Internet<URL: http://www.kumikomi.net/article/explanation/2002/04usbgdv/0 .1html>
In order to work a unit as a dual role device, however, hardware and software for realizing the USB-OTG function is required, that is, it is necessary to mount a receptacle for exclusive use and install a predetermined protocol such as SRP (Session Request Protocol) and HNP (Host Negotiation Protocol). Accordingly, there is a problem of increasing the cost of the unit.